Optical disk and optical disk apparatus

ABSTRACT

An optical disk comprises a substrate, and a recording layer which is formed on the substrate and on which information is recorded at specific pitches in the form of pit trains, wherein the information is reproduced by projecting a light beam via an object lens, and when the wavelength of the light beam is λμm and the numerical aperture of the object lens is NA, the track pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm, and each of the pits has a trapezoid cross section whose upper width is in the range of (0.3 to 0.5)×/NA/1.14 μm and whose lower width is in the range of (0.2 to 0.32)×λ/NA/1.14 μm or whose inner wall has an angle of 30° to 70°.

This is a continuation of application Ser. No. 08/304,849, filed Sep.13, 1994, now U.S. Pat. No. 5,459,712.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical disk on which information isrecorded in pits with high density and an optical disk apparatuscontaining the optical disk and a playback optical system.

2. Description of the Related Art

With the recent advances in image digital signal processing techniquesand moving-picture compression techniques, the latter of which have beendeveloped by such a standardizing organization as the MPEG (MovingPicture Image Coding Experts Group), there is a growing expectation ofthe advent of an optical disk capable of reproducing moving-pictureinformation such as a movie for two hours and being the same size as aCD (compact disk) in place of a VTR or laser disk. The recordingcapacity required to record two hours of moving-picture information inthe form of analog video signals by a standard TV system such as NTSC ason the laser disk, amounts to 80 Gbyte including sound. Use ofmoving-picture compression techniques prescribed by a standardizedmethod called MPEG-2, for example, requires as small a capacity asnearly 4 Gbyte even for a picture quality as good as a highpicture-quality VTR such as S-VHS. The 4-Gbyte disk has been put intopractical use in the form of a 300-mm diameter write-once read-manyoptical disk. As more and more optical disks will be used in homes inthe future, it is needed to achieve an easy-to-use 120-mm diameter diskwhich has the same size and almost the same capacity as the CD.

The capacity of the CD format presently available as the music CD or theCD-ROM is 790 Mbyte at the maximum (when the linear velocity is 1.2m/s). The capacity of this order can store only 24 minutes of compressedmoving-picture information by MPEG-2. Thus, to store two hours ofcompressed moving-picture information by MPEG-2 with the CD size, therecording density must be made five times as high as that of the CD. Inthe current CD format, the substrate thickness is 1.2 mm, the trackpitch is 1.6 μm, the pit pitch is 1.66 μm when the linear velocity(relative velocity between light beam and disk=disk's circumferentialvelocity) is 1.2 m/s, the bit length is 0.59 μm, and the modulationmethod is EFM (eight to fourteen modulation). In the playback opticalsystem, the playback semiconductor laser, or the laser diode (LD) has awavelength of 780 nm, the object lens has an NA (numerical aperture) of0.45, and the beam spot has a diameter of 1.4 μm. The beam spot diameteris selected mainly from the standpoint of avoiding the effect ofcrosstalk between adjacent tracks.

To increase the recording density of the optical disk requirestechniques for forming small pits in the disk and those for making thebeam spot size small the optical disk in the playback optical system.Concerning techniques for forming pits, for example, an optical diskmatrix recording technique using Kr ion laser light (ultraviolet rays)with a wavelength of 351 nm has been proposed (The 1993 Autumn NationalConvention of the Applied Physics Society, 28-SF-2). This techniquemakes it possible to form smaller pits than a conventional Ar ion laser.In the playback optical system, by making the wavelength of the playbacklaser beam shorter and increasing the NA, the beam spot diameter can bemade smaller. Actually, however, with conventional techniques used in CDplayers, even if a short wavelength light source such as a red laserdiode were used, the capacity would be increased by 1.5 times at most.With such an increase in the capacity, it cannot be expected to increasethe capacity by five times that of an ordinary CD, which is what isrequired to record two hours of compressed moving-picture information.

As described above, with the conventional optical disk techniques, toavoid the problem of crosstalk between adjacent tracks, the track pitchand pit pitch are set larger than the beam spot diameter of theplay-back light beam. As a result, only by making the wave-length ofplayback light beam shorter and increasing the NA of the object lens,the recording density cannot be raised to the extent that the capacityrequired to store two hours of compressed moving-picture information byMPEG2 with the CD size, for example.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an optical disk and anoptical disk apparatus which can lessen crosstalk between adjacenttracks to the extent that there is no problem in practical use, even ifthe track pitch and pit pitch are smaller than the beam spot diameter ofthe playback light beam, and which achieves a higher density and agreater capacity than in the prior art.

According to the present invention, there is provided an optical diskcomprising a substrate and a recording layer which is formed on thesubstrate and on which information is recorded at specific pitches inthe form of pit trains, wherein the information is reproduced byprojecting a light beam via an objective lens, and when the wavelengthof the light beam is λ μm and the numerical aperture of the objectivelens is NA, the track pitch is set in the range of (0.72 to0.8)×λ/NA/1.14 μm. Each of the pits has a trapezoidal cross sectionwhose upper width is in the range of (0.3 to 0.5)×λ/NA/1.14 μm and whoselower width is in the range of (0.2 to 0.32)×λ/NA/1.14 μm.

According to the present invention, there is provided an optical diskapparatus comprising an optical disk comprising a substrate and arecording layer which is formed on the substrate and on whichinformation is recorded at specific pitches in the form of pit trains,an objective lens provided so as to face the optical disk, means forprojecting a light beam onto the optical disk via the object lens, andmeans for sensing the reflected light of the light beam projected on theoptical disk by the projecting means to reproduce the informationrecorded on the optical disk, wherein when the wave-length of the lightbeam is λ μm and the numerical aperture of the objective lens is NA, thetrack pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm. Each ofthe pits has a trapezoid cross section whose upper width is in the rangeof (0.3 to 0.5)×λ/NA/1.14 μm and whose lower width is in the range of(0.2 to 0.32)×λ/NA/1.14 μm.

According to the present invention, there is provided an optical diskcomprising a substrate and a recording layer which is formed on thesubstrate and on which information is recorded at specific pitches inthe form of pit trains, wherein the information is reproduced byprojecting a light beam via an object lens, and when the wavelength ofthe light beam is λ μm and the numerical aperture of the object lens isNA, the track pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm.Each of the pits has a trapezoid cross section whose upper width is inthe range of (0.3 to 0.5)×λ/NA/1.14 μm and whose inner wall has an angleof 30° to 70°.

According to the present invention, there is provided an optical diskapparatus comprising an optical disk comprising a substrate and arecording layer which is formed on the substrate and on whichinformation is recorded at specific pitches in the form of pit trains,an objective lens provided so as to face the optical disk, means forprojecting a light beam onto the optical disk via the objective lens,and means for sensing the reflected light of the light beam projected onthe optical disk by the projecting means to reproduce the informationrecorded on the optical disk. When the wavelength of the light beam is λμm and the numerical aperture of the object lens is NA, the track pitchis set in the range of (0.72 to 0.8)×λ/NA/1.14 μm. Each of the pits hasa trapezoidal cross section whose upper width is in the range of (0.3 to0.5)×λ/NA/1.14 μm and whose inner wall has an angle of 30° to 70°.

By setting various parameters of the pit shape at the above-describedvalues, the amount of crosstalk between adjacent tracks is suppressed toless than -20 dB, which must be met to restore the original informationfrom the reproduced signal, and the playback signal level and the levelof the push-pull signal for tracking are maintained sufficiently.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a diagrammatic view to help explain the shape of a pit in anoptical disk according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of the playback optical system in anoptical disk apparatus;

FIG. 3 is a diagrammatic view of the pit arrangement on the optical diskfor calculating the levels of the playback and push-pull signals sensedfrom the optical disk;

FIG. 4 shows the relationship between the playback signal level and thepush-pull signal level obtained from calculations using the pit size inthe track width direction and the pit depth as parameters;

FIG. 5 is a diagrammatic view of the pit arrangement on the optical diskused to evaluate the MTF of the playback optical system and crosstalkbetween adjacent tracks;

FIG. 6 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the upper width Wm and lower width Wi of the pit are changedvariously;

FIG. 7 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the conditions are the same as in FIG. 6 except that λ is set at0.650 μm;

FIG. 8 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the upper width Wm and the angle θ of the pit's inner wall arechanged variously;

FIG. 9 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the conditions are the same as in FIG. 8 except that the upperwidth Wm of pot 10 is fixed at 0.35 μm;

FIG. 10 shows the dependence of the playback optical system's MTF andthe characteristics of crosstalk between adjacent tracks on the tilt;

FIG. 11 shows the dependence of the NA of the object lens on the tiltwhen an optical disk whose substrate thickness is 1.2 mm is used;

FIG. 12 shows the dependence of the NA of the object lens on the tiltwhen an optical disk whose substrate thickness is 0.6 mm is used;

FIG. 13 shows a cross sectional outline of an actual pit;

FIG. 14 shows a relationship between the groove width of an actual pitand the groove width of a model pit;

FIG. 15 shows a relationship between between an angle of inclination ofan actual pit and that of a model pit;

FIGS. 16A and 16B are a perspective view and a sectional view of anoptical disk according to an embodiment of the present invention; and

FIG. 17 is a block diagram of an optical disk apparatus according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explanation of an embodiment, the basic concept of the presentinvention will be described.

To make the density of an optical disk higher, the spot diameter of theplayback light beam must be made smaller. To do this, it is essential tomake the wavelength of the playback laser diode shorter and increase theNA of the objective lens. Laser diodes (self-pulsation-type laserdiodes) of a low-noise type whose wavelength is 0.685 μm and whoseoutput is several milliwatts have already been put into practical use.Laser diodes whose wavelength is 0.650 μm are getting close to practicaluse.

The NA of the objective lens is limited by ease of making a lens and thetilt angle between the lens and the disk. The smaller the lens load (thethinner the optical disk substrate, the smaller the lens load) and thesmaller the NA, the easier it is to make the object, lens. For anobjective lens whose NA is nearly 0.6, it is possible to make the beamspot diameter smaller even with a single nonspherical lens. However,with an objective lens used in the playback optical system for theoptical disk, coma aberration occurs due to the tilt between the opticaldisk and the playback light beam caused by the tilt of the optical diskor the tilt of the optical axis of the objective lens.

Specifically, an attempt to make the NA of the objective lens larger inorder to make the spot size of the playback light beam smaller permitsthe aberration of the objective lens to increase sharply due to the tiltbetween the optical disk and the playback light beam. As the aberrationof the objective lens becomes larger, the amount of crosstalk betweenadjacent tracks increases accordingly, and the playback resolving powerdecreases. The thinner the optical disk substrate, the smaller theeffect of the tilt. In the Japanese Journal of Applied Physics, Vol. 32(1993), pp. 5402-5405, changes in the shape of the spot of the playbacklight beam corresponding to the tilt when the substrate thickness is 1.2mm, the same as the CD, and 0.6 mm, with a wavelength of 0.690 μm andNA=0.6 are explained. According to this description, when the substratethickness is 1.2 mm, a tilt of 5 mrad lowers the center strength of thebeam spot as much as 10% and causes a rise in the side lobe andaberration, which contributes to crosstalk. In contrast, when thesubstrate thickness is 0.6 mm, the substrate can withstand the tiltsranging up to 10 mrad.

FIGS. 11 and 12 show the results of calculating the tilt characteristicswhen the substrate thickness (t) is 1.2 mm and 0.6 mm using the NA as aparameter. The abscissa represents the angle of tilt and the ordinateindicates the normalized peak intensity of the playback signal. Thewavelength (λ) of the playback light beam is assumed to be 0.690 μm.With the substrate thickness being 0.6 mm and the NA=0.6, the tilt is9.5 mrad and the peak intensity of the playback signal decreases by 10%.When the substrate thickness is 1.2 mm, the NA=0.49. Specifically, bychanging the substrate thickness from 1.2 mm (the conventional CD'sthickness) to 0.6 mm, the NA can be increased from 0.49 to 0.6, andconsequently the surface recording density can be increased by 1.5times. Because the spot size is proportional to λ/NA and the surfacerecording density is proportional to 1/2² of the spot size, this gives(0.6/0.49)², which means that the area recording density is 1.5 times ashigh as that of the conventional CD.

However, just making the substrate thickness thinner can cause thesubstrate to warp substantially due to temperature or humidity. The warpof the substrate contributes mainly to the tilt. To avoid this, it ismost effective to make the optical disk double-sided as the laser disk,or to give the optical disk a symmetrical structure with respect to thefront and back. In that case, it is possible to record information onboth sides. With a single-structure optical disk like the conventionalCD, since an aluminum reflective film or protective film is formed onone side of the substrate, the substrate has an asymmetrical moistureabsorption with respect to the front and back, and thus tends to warpeasily. A double-sided optical disk cancels the distortion of thesubstrate due to moisture absorption, thereby preventing a large tiltfrom occurring.

The evaluation results described above show that if a combination of alaser diode with a wavelength of 0.685 μm, a 0.6-mm thick substrate, andan objective lens with an NA=0.6 are used, the wavelength will beshortened from 0.780 μm to 0.685 μm and the NA will grow larger from0.45 to 0.6, so that the recording density can be made about 2.3 timesas large as that of the conventional CD format even by conventional CDdesign techniques. Specifically, because the spot size is proportionalto λ/NA, this gives (0.685/0.6)/(0.780/0.45), meaning that the recordingdensity is about 2.3 times as high as that of the conventional CD.However, to achieve the capacity required to record two hours ofcompressed moving-picture information by MPEG2 with the CD size, it isnecessary to make the recording density (capacity) about five times aslarge as that of the conventional CD format.

According to the present invention, there is provided an optical diskwhich enables the track pitch to be made much smaller in order toachieve a much higher density and greater capacity, while assuring thelow crosstalk characteristics and the sufficient signals levels of theplayback signal and the push-pull signal by optimizing the pit shape ofthe same beam spot size as described above. Hereinafter, the pit shapein the present invention will be described in detail.

FIG. 1 is an explanatory diagram of the shape of a pit in an opticaldisk according to the present invention. As shown in the figure, theshape of a pit is approximated by a shape with a trapezoidal crosssection. The inner wall 11 of the pit 10 is inclined downward and itsbottom portion 12 is almost flat. Numeral 13 indicates the cross sectionof the pit 10 along the radius of the optical disk (the track widthdirection); 14 the cross section along the circumference of the opticaldisk (the track direction); Wm the size of the top of pit 10 across thetrack width (hereinafter, the upper width); wi the size of the bottom ofthe pit 10 across the track width (hereinafter, the lower width); hm thedepth of the pit 10, Zm the length of the pit 10 along the track; and θrepresents the angle of the inner wall of the pit 10 (the angle that theinner wall forms with respect to the surface of the optical disk).

FIG. 2 shows a model of the playback optical system of the optical diskapparatus used in the analysis. The figure shows the incident lightdistribution 20 (V1 (x,y)) of playback light beam, incident light 21, apolarization beam splitter (or half mirror) 22 for separating theincident light 21 from the reflected light 26, an objective lens 23 witha numerical aperture of NA, the distribution 24 (V2(x,y)) of focusedlight (beam spot) by the object lens on the recording surface (pitsurface) of the optical disk, an optical disk 25 with a complexreflectivity of r2(x,y), the reflected light 26, and the distribution 27(V3(x,y)) of the reflected light 26 on a photosensor.

FIG. 3 diagrammatically shows the pit arrangement on the optical diskused to calculate the levels of the playback signal and the push-pullsignal, where the track pitch (the pitch of a pit across the trackwidth) is Pt and the pit pitch (the pit pitch along the track) is Pmy.The beam spots 30 and 31 of the playback light beam represent spot A atthe center of a pit and spot B between pits, respectively. The amplitudeof the playback signal is represented as |S(A)-S(B)|, where S(A) andS(B) indicate the output signals of the photosensor when the beam spotis positioned at (A) and (B), respectively. Lines 32 and 33 show thepositions in which the push-pull signal (the difference signal betweenthe output signals of the split photosensors arranged along the track inat least two sensing areas) is obtained in an area (C) with pits and anarea (D) without pits. Those push-pull signals each have the average p-pvalues in area C and area D.

FIG. 4 shows the results of calculating the levels of the playbacksignal and the push-pull signal using the size of a pit across the trackwidth and the pit depth as parameters with a playback laser beamwave-length of 0.685 μm, NA=0.6, Zm=0.5 μm, Pmy=1 μm, and Pt=0.72 μm.The values of the beam filling factors A/W(X) and A/W(Y) of the playbacklight beam across the track width (X) and along the track (Y) are shownin the figure. As seen from the figure, the levels of the playbacksignal and the push-pull signal do not depend largely on the shape of apit except the case where Wm=0.3 and Wi=0.2. There is no such pit depthas brings the levels of the playback signal and the push-pull signal tothe maximum level simultaneously. To minimize a decrease in thepush-pull signal level and obtain the maximum playback signal level, itis desirable from FIG. 4 that the pit depth should be approximately λ/5,preferably in the range of λ/4.2 to λ/5.2. More specifically, the pithas a depth in a range of 1/4.2×λ/n to 1/5.2×λ/n, where n is arefractive index of the substrate.

FIG. 5 diagrammatically shows an MTF of the playback optical system andthe pit arrangement on the optical disk used to evaluate crosstalkbetween adjacent tracks. In the figure, the spots 50 and 51 of theplayback light beam represent a spot at the center (A) of a pit and aspot passing through position (B) a distance of td away from the centerof a pit. MTF is expressed by the power of the basic frequency componentof the output signal from the photosensor obtained when the beam spotpasses through the center of a pit. Crosstalk is expressed by the powerof the basic frequency component of the output signal from thephotosensor obtained when the beam spot passes through position B.

FIG. 6 shows the MTF and the crosstalk characteristics when the trackpitch Pt is fixed at 0.72 μm, the depth hm of the pit 10 is fixed at 0.2μm, and the upper width wm and lower width wi of the pit 10 are changedvariously, with the spatial frequency on the abscissa and MTF andcrosstalk on the ordinate. The values of beam packing factors A/W(X) andA/W(Y) of the playback light beam across the track width (X) and alongthe track (Y) are shown in the figure. In the figure, the MTF produces adifference of 1 to 2 dB depending on the pit shape, which is not toolarge. In contrast, it can be seen that crosstalk changes greatly withthe pit shape.

FIG. 7 shows the MTF and the crosstalk characteristics under the sameconditions as in FIG. 6 except that λ is set at 0.650 μm.

FIG. 8 shows the MTF and the crosstalk characteristics when the trackpitch Pt is fixed at 0.72 μm, the depth hm of the pit 10 is fixed at 0.2μm, and the upper width Wm and the angle 8 of the inner wall 11 of thepit 10 are changed varied, with the spatial frequency on the abscissaand MTF and crosstalk on the ordinate. The values of beam packingfactors A/W(X) and A/W(Y) of the playback light beam across the trackwidth (X) and along the track (Y) are shown in the figure.

FIG. 9 shows the MTF and the crosstalk characteristics under the sameconditions as in FIG. 8 except that the upper width Wm of the pit 10 isfixed at 0.35 μm and only the angle θ of the inner wall 11 of the pit 10is varied.

It is assumed that the 4/9 modulation method, whose efficiency is higherthan that of EFM used in the conventional CD, and the (3, 17) RLL(Run-Length Limited) method are used as a modulation method ofinformation recorded on the optical disk. This is a coding method ofconverting the 4-bit original information into 9 bits and limiting thenumber of consecutive 0s to between 3 to 17. The density ratio isimproved by 20% as compared with EFT including DCC. In this case, if theshortest pit length is 0.48 μm, the maximum pit length will be 2.16 μm.Therefore, it is necessary to pay attention to crosstalk due to lowfrequency components when the longest pit is sensed. Although thecrosstalk characteristics shown in FIGS. 6 to 9 are for no tilt, tiltsactually have to be taken into account. FIG. 10 shows the MTF and thecrosstalk characteristics when tilts are taken into account. As can beseen from the figure, when tilts are taken into account, the MTF almostremains unchanged, but the amount of crosstalk increases, and theconditions for determining the parameters of a pit become more strict.

In the system design of an optical disk apparatus, if a tilt due to thewarp of the optical disk itself and a tilt due to the apparatus areconsidered to be 5 mrad and 3 mrad, respectively, a tilt of about 8 mradin total must be tolerated. According to the simulation in FIG. 10, theamount of crosstalk can be suppressed to values less than -20 dBrequired in practical use in the tilt range of ±10 mrad for the samespatial frequency. This shows that a wavelength of 0.685 μm and a trackpitch of 0.72 μm are reasonable.

The evaluation of FIGS. 6 to 9 shows that the amount of crosstalk (thedifference in MTF value between the MTF characteristic and the crosstalkcharacteristic) is 20 dB or less until Wm=0.45 μm in the case where thetrack pitch shown in FIGS. 6 and 8 is 0.72 μm. In contrast, when Wm=0.5μm, the amount of crosstalk at low frequencies increases rapidly,exceeding -20 dB. The MTF characteristics are relatively good untilWm=0.3 μm, but deteriorates sharply when wm is less than 0.3 μm.Therefore, the range of Wm=0.3 μm to 0.45 μm is reasonable.

The results mentioned above show that when the pit shape across thetrack width is standardized with a wavelength of 0.685 μm and NA=0.6(i.e., λ/NA=1.14), it is desirable that the upper width of a pit shouldbe (0.3 to 0.45 )×λ/NA/1.14 μm, and the lower width of the pit be (0.2to 0.25 )×λ/NA/1.14 μm, or that the upper width Wm should be (0.3 to0.45)×λ/NA/1.14 μm and the angle θ of the inner wall should be in therange of 50° to 70°. Specifically, when the track pitch Pt is selectedin the range of (0.72 to 0.8 )×λ/NA/1.14 μm and the track pitch is madesmaller than the beam spot diameter of the playback light beam,selecting the upper width Wm and lower width wi of the pit or the upperwidth Wm of the pit and the angle θ of the inner wall in the aboveranges enables the amount of crosstalk to be suppressed to values lessthan -20 dB required in practical use in the tilt range of ±10 mradexpected in an actual optical disk apparatus, thereby achieving aremarkable improvement in the recording density. As a result, bycombining these track pitch and pit shape, the laser diode with awavelength of 0.685 μm, for example, as mentioned earlier, the 0.6-mmthick substrate, the object lens with NA=0.6, the subject of recordingtwo hours of compressed moving-picture information by MPEG with the CDsize can be achieved easily.

The parameters used in the explanation of the invention are obtainedthrough calculations on the assumption that the pit is in the form of anideal trapezoid. Actually, however, the pit does not take the form of anaccurate trapezoid, but curves at its corners as shown in FIG. 13.Therefore, the parameters for the ideally trapezoidal pit, or the modelpit, differ from those for the actual pit. FIG. 14 shows the differencebetween the bottom groove width wi and the top groove width wg for themodel pit and the actual pit. As seen from FIG. 14, the value of wi forthe model pit ranges from 0.2 to 0.32 μm; the value of wg for the modelpit ranges from 0.3 to 0.45 μm, whereas the value of wg for the actualpit varies from 0.3 to 0.5 μm. Furthermore, the angle of inclination θof the pit is as follows. As shown in FIG. 15, an angle of inclinationof the mode pit is in the range of 50° to 70°, whereas that of theactual pit in the range of 30° to 60°.

Hereinafter, the structure of an optical disk according to the presentinvention will be described. FIGS. 16A and 16B are a perspective viewand sectional view of a double-sided optical disk 100, respectively. Onesurface of each of transparent substrates 101 and 102 is embossed withpits made of light-transmitting resin such as polycarbonate or acrylicresin and is coated with a reflecting film 103 and a reflecting film 104(e.g., of aluminum), respectively. On these films, protective films 105and 106 are formed. The thickness of the transparent substrates 101 and102 is 0.6 mm. The transparent substrates 101 and 102, whose protectivefilms 105 and 106 are allowed to face each other, are laminated togetherwith an adhesion layer 107 with a thickness of several tens of μm madeof a thermoset adhesive. In the center of the optical disk 100, a hole108 is made for clamping. Around the hole, a clamping zone 109 isprovided. A playback light beam 110 is emitted from a laser diode (notshown), passes through the playback optical system, enters an opticaldisk 100 via an object lens 111 from the transparent substrates 101 and102, and is focused with a small beam spot on the reflecting films 103and 104.

FIG. 17 shows an example of an optical disk apparatus which reproducescompressed moving-picture information by using the above-mentionedoptical disk 100. Because the optical disk 100 uses the substrates 101and 102 which are as thin as 0.6 mm and consequently is less immune todust or dirt on their surfaces than a CD using a 1.2-mm thick substrate,the desk 100 is housed in a cartridge 200. By housing the optical disk100 in the cartridge 200, attention need not be paid to the way ofholding the disk, dust, fingerprints, etc. as with CDs, which is helpfulin handling and carrying. When the disk is exposed as is a CD, theability to correct errors must be determined, taking into account anunexpected accident such as a flaw. Use of the cartridge 200, however,makes such a consideration unnecessary. Therefore, it is possible to usethe LDC read Solomon error correction technique in sectors as used in arecordable optical disk. As a result of this, for example, when anoptical disk is formatted in units of 2 kbyte to 4 kbyte, the recordingefficiency can be increased by more than 10% as compared with the CD.

When the 4/9 modulation method is used as a modulation method for theinformation recorded on the optical disk 100, the track pitch on theoptical disk 100 is 0.72 μm, and the pit pitch is 0.96 μm, it isexpected that the pit density ratio is 3.84 times as high as theconventional CD format, the modulation efficiency is increased by 20%,and the format efficiency is increased by 10%. Consequently, thecapacity can be expected to increase by a total of 5.1 times. Asdescribed earlier, when moving-picture information such as a movie isreproduced with a picture quality as high as S-VHS, this requires a rateof 4.5 Mbps including sound, so that the capacity required for two hoursof reproduction is 4 Gbyte. Because of the aforementioned capacityincrease by 5.1 times, the 4-Gbype capacity can be realized on one sideof the disk. Furthermore, as shown in FIGS. 16A and 16B, a singledouble-sided optical disk alone enables four hours of recording at amaximum.

In FIG. 17, the optical disk 100 is chucked by a tapered cone 220 whichis rotated by a spindle motor 201. The spindle motor 201 is driven by aspindle motor driver circuit 202. The playback optical system isconstructed as follows.

An objective lens 203 is placed so as to face the optical disk 100. Theobjective lens 203 can be moved along the optical axis by a focus coil204 and across the track width by a tracking coil 205. The wavelength ofa laser diode 207 driven by a laser diode (LD) driver 206 is 0.685 μm.The light beam emitted from the laser diode 207 is made into parallelluminous flux by a collimate lens 208 and then enters a polarizationbeam splitter 209. The light beam emitted from the laser diode 207 hasgenerally an elliptic far field pattern. Therefore, when a round patternis needed, a beam shaping prism has only to be placed after thecollimate lens 208. The light beam passed through the polarization beamsplitter 209 is focused by the objective lens 203 onto the optical disk100.

The light reflected by the reflecting film on the optical disk 100passes back through the objective lens 203 in the opposite direction tothe incident light beam, is reflected by the polarization beam splitter209, and enters a photosensor 212 via the sensing optical systemcomposed of a condenser lens 210 and a cylindrical lens 211. Thephotosensor 212 is, for example, a 4-quadrant photosensor. The foursense outputs of the photosensor are input to an amplifier array 213containing an amplifier and an adder-subtracter, which produces a focuserror signal, tracking error signal, and playback signal. The trackingerror signal is obtained by, for example, a push-pull technique in theform of a push-pull signal as described earlier. The focus error signaland tracking error signal are supplied to the focus coil 204 and thetracking coil 205 via a servo controller 214. As a result of this, theobjective lens 203 is moved along the optical axis and across the trackwidth, thereby focusing the light beam onto the surface of thereflecting film serving as the recording surface of the optical disk100, and tracking the target track.

The playback signal from the amplifier array 213 is input to a signalprocessing circuit 215, which binarizes the input and then senses datapulses. The sensed data pulses are inputted to a disk controller 16,which decodes the format, corrects errors, and then supplies theresulting signal as a bit stream of moving-picture information to anMPEG2 decoder/controller 217. Because the data obtained by compressing(encoding) the moving-picture information according to the MPEG2standards is recorded on the optical disk 100, the MPEG2decoder/controller 217 expands (decodes) the bit stream input toreproduce the original moving-picture information. The reproducedmoving-picture information is supplied to a video signal generatorcircuit 218, which adds a blanking signal etc. to produce a video signalin a specific television format. The techniques related to MPEG2 havebeen disclosed in U.S. Pat. No. 5,317,397 and U.S. Pat. application Ser.No. 08/197,862.

As explained above, the optical disk according to the present inventionhas such an optimal pit shape (the upper and lower widths of a pit orthe upper width of a pit and the angle of the pit's inner wall) as makesit possible to set the track pitch to a smaller value than the spotdiameter of the playback light beam and decrease crosstalk betweenadjacent tracks to a level required for practical use. As a result, withthe optical disk, the track density can be made by about 1.5 times ashigh as the conventional CD and the sufficient levels of the playbacksignal and the push-pull signal used for tracking can be assured.

Accordingly, with the present invention, as shown in the embodimentsdescribed above, the capacity about five times that of the conventionalCD can be realized even using the normal CD size, for example. Inaddition, 4 Mbps of compressed moving-picture information with a picturequality as good as that of a high quality VTR, including sound, can bestored for two hours, which is very useful in practical use.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. An optical disk comprising:a substrate havinginformation recorded thereon with a specific track pitch, saidinformation being recorded as a plurality of pit trains, each of saidpit trains including a plurality of pits; a reflecting layer formed onsaid substrate; wherein said information is reproduced by projecting alight beam via an objective lens onto said optical disk; and whereinwhen the wavelength of said light beam is λ μm and the specific trackpitch is in the range of (0.72 to 0.8)×(λ/NA)1.14 μm, and each of saidpits has a substantially trapezoidal cross section having an upper widthin the range of (0.3 to 0.5)×(λ/NA)/1.14 μm, a lower width in the rangeof (0.2 to 0.32)×(λ/NA)/1.14 μm, and a depth in a range of (1/4.2×λ/n)to (1/5.2×λ/n), where n is a refractive index of said substrate and NAis a numerical aperture of said objective lens.
 2. An optical diskaccording to claim 1, wherein said substrate is composed of a pair oftransparent substrates facing each other, and said reflecting layer isformed of a pair of reflecting films each of which is formed betweensaid pair of transparent substrates and on each of which information isrecorded.
 3. An optical disk according to claim 2, wherein on saidreflecting films, a pair of protective films is formed, which areallowed to face each other and are laminated together.
 4. An opticaldisk according to claim 3, which includes an adhesion layer formedbetween said protective films to adhere them to each other.
 5. Anoptical disk according to claim 1, wherein said substrate is made of oneof polycarbonate and acrylic resin.
 6. An optical disk apparatusaccording to claim 1, which further includes a secondary substrate and asecondary reflecting layer formed on said secondary substrate.
 7. Anoptical disk apparatus according to claim 1, which further comprises aprotective film formed on said reflecting layer.
 8. An optical diskapparatus comprising:an optical disk including a substrate havinginformation recorded thereon with a specific track pitch, saidinformation being recorded as a plurality of pit trains, each of saidpit trains including a plurality of pits and a reflecting layer formedon said substrate; an objective lens disposed so as to face said opticaldisk; means for projecting a light beam onto said optical disk via saidobjective lens; and means for sensing reflected light of the light beamreflected from said optical disk to reproduce the information recordedon said optical disk; wherein when said light beam has a wavelength λ μmand said objective lens has a numerical aperture NA, said track pitch isin the range of (0.72 to 0.8)×λ(λ/NA)/1.14 μm, and each of said pits hasa substantially trapezoidal cross section having an upper width in therange of (0.3 to 0.5)×(λ/NA)/1.14 μm, a lower width in the range of (0.2to 0.32)×(λ/NA)/1.14 μm, and a depth in a range of (1/4.2×λ/n) to(1/5.2×λ/n), where n is a refractive index of said substrate.
 9. Anoptical disk according to claim 8, wherein said substrate is made of oneof polycarbonate and acrylic resin.
 10. An optical disk apparatusaccording to claim 8, wherein said sensing means produces a push-pullsignal representing a difference between signals sensed in at least twoareas along the track and a playback signal, and the depth of said pitcorresponds to a depth, which enables the two of said push-pull signalsand said playback signal to have large levels.
 11. An optical diskapparatus according to claim 8, wherein said sensing means expandsinformation compressed according to MPEG2 and recorded on said opticaldisk, and reproduces original information according to MPEG2.
 12. Anoptical disk comprising:a substrate having information recorded thereonat a specific track pitch, said information being in the form of aplurality of pit trains, each of said pit trains including a pluralityof pits; and a reflecting layer formed on said substrate; wherein saidinformation is reproduced by projecting a light beam via an objectivelens, and when said light beam has a wavelength λ μm and said objectivelens has a numerical aperture NA, said specific track pitch is in therange of (0.72 to 0.8)×(λ/NA)/1.14 μm, and each of said pits has asubstantially trapezoidal cross section having an upper width in therange of (0.3 to 0.5)×(λ/NA)/1.14 μm, an inner wall inclined at an angleof 30° to 60°, and a depth in a range of (1/4.2×λ/n) to (1/5.2×λ/n),where n is a refractive index of said substrate.
 13. An optical diskaccording to claim 12, wherein said substrate is composed of a pair oftransparent substrates facing each other, and said reflecting layer isformed of a pair of reflecting films each of which is formed betweensaid pair of transparent substrates and on each of which information isrecorded.
 14. An optical disk according to claim 13, wherein a pair ofprotective films is formed on said reflecting films, said protectivefilms being allowed to face each other and being laminated together. 15.An optical disk according to claim 14, which includes an adhesion layerformed between said protective films to adhere them to each other. 16.An optical disk according to claim 12, wherein said substrate is made ofone of polycarbonate and acrylic resin.
 17. An optical disk apparatusaccording to claim 12, which further includes a secondary substratecarrying information and a secondary reflecting layer formed on saidsecondary substrate.
 18. An optical disk apparatus according to claim17, further comprising a pair of protective films formed on saidreflecting layer and said secondary reflecting layer, respectively. 19.An optical disk apparatus according to claim 12, further comprising asecondary substrate and a secondary reflecting layer formed on saidsecondary substrate.
 20. An optical disk apparatus according to claim12, further comprising a protective film formed on said reflectinglayer.
 21. An optical disk apparatus comprising:an optical diskincluding a substrate having information recorded thereon with aspecific track pitch, said information being recorded as a plurality ofpit trains, each of said pit trains including a plurality of pits and areflecting layer formed on said substrate; an objective lens disposed soas to face said optical disk; means for projecting a light beam ontosaid optical disk via said objective lens; and means for sensing thereflected light of the light beam projected on said optical disk by saidprojecting means to reproduce the information recorded on said opticaldisk; wherein said light beam has a wavelength λ μm and said objectivelens has a numerical aperture NA, said specific track pitch is in therange of (0.72 to 0.8)×(λ/NA)/1.14 μm, and each of said pits has asubstantially trapezoidal cross section having an upper width in therange of (0.3 to 0.5)×(λ/NA)/1.14 μm, an inner wall inclined at an angleof 30° to 60°, and a depth in a range of (1/4.2×λ/n) to (1/5.2×λ/n),where n is a refractive index of said substrate.
 22. An optical diskapparatus according to claim 21, wherein said sensing means produces apush-pull signal representing a difference between signals sensed in atleast two areas along the track and a playback signal, and the depth ofsaid pit corresponds to a depth that enables the two of said push-pullsignals and said playback signal to have large levels.
 23. An opticaldisk apparatus according to claim 21, wherein said sensing means expandsinformation compressed according to MPEG2 and recorded on said opticaldisk, and reproduces original information according to MPEG2.
 24. Anoptical disk comprising:a pair of transparent substrates with surfacesfacing one another, each of said transparent substrates havinginformation recorded thereon at a specific track pitch, each of said pittrains including a plurality of pits, wherein said information isreproduced by projecting a light beam via an objective lens; a pair ofreflecting layers coated on said facing surfaces of said pair ofsubstrates, respectively; a pair of protective layers formed on saidpair of reflecting layers respectively; and an adhesive layer formedbetween said protective layers to adhere said pair of protective layersto one another; wherein when said light beam has a wavelength λ μm andsaid objective lens has a numerical aperture NA, said specific trackpitch is in the range of (0.72 to 0.8)×(λ/NA)/1.14 μm, and each of saidpits has a substantially trapezoidal cross section having an upper widthin the range of (0.3 to 0.5)×λ/NA)/1.14 μm, a lower width in the rangeof (0.2 to 0.32)×(λ/NA)/1.14 μm, and a depth in a range of (1/4.2×λ/n)to (1/5.2×λ/n), where n is a refractive index of said substrate.
 25. Anoptical disk comprising:a pair of transparent substrates with surfacesfacing one another, each of said transparent substrates havinginformation recorded thereon at a specific track pitch, each of said pittrains including a plurality of pits, wherein said information isreproduced by projecting a light beam via an objective lens; a pair ofreflecting layers coated on said facing surfaces of said pair ofsubstrates, respectively; a pair of protective layers formed on saidpair of reflecting, respectively; and an adhesive layer formed betweensaid protective layers to adhere said pair of protective layers to oneanother; wherein when said light beam has a wavelength λ μm and saidobjective lens has a numerical aperture NA, said specific track pitch isin the range of (0.72 to 0.8)×(λ/NA)/1.14 μm, and each of said pits hasa substantially trapezoidal cross section having an upper width in therange of (0.3 to 0.5)×(λ/NA)/1.14 μm, an inner wall inclined at an angleof 30° to 60° and a depth in a range of (1/4.2×λ/n) to (1/5.2×λ/n),where n is a refractive index of said substrate.